Method and system for medical tracking using a plurality of camera positions

ABSTRACT

A method for tracking trackable objects using a medical tracking device, the tracking device being a camera or an EM transmitter, during a medical workflow comprising a plurality of workflow steps, wherein each trackable object has at least one marker and the method comprises the steps of: —acquiring a set of camera positions, wherein each tracking device position is associated with at least one workflow step; —identifying a workflow; —sequentially and automatically moving the tracking device to the camera positions associated with the workflow steps; and—performing a tracking step only when the tracking device is in a fixed position.

The present invention relates to a method and a system for medicallytracking trackable objects using a tracking device during a medicalworkflow comprising a plurality of workflow steps. Each trackable objecthas at least one marker.

In known medical tracking systems, a tracking camera, in particular a 3Dtracking camera, observes a certain spatial region in order to detectmarkers. However, markers cannot be detected if the line of sightbetween the camera and the marker is obstructed. In document DE 10 2008055 918 A1, the tracking camera is moved during the tracking process inorder to avoid an obstruction in the field of view. However, thetracking accuracy is reduced while the tracking camera is moving. Theproblem equally arises for medical tracking systems in which an EMtransmitter transmits an EM signal (electromagnetic signal) which isreceived by a marker, while the received signal is analysed in order todetermine the position of the marker.

This problem is solved by the subject-matter of any of the appendedindependent claims. Advantages, advantageous features, advantageousembodiments and advantageous aspects of the present invention aredisclosed in the following and contained in the subject-matter of thedependent claims. Different advantageous features can be combined inaccordance with the invention where technically expedient and feasible.In particular, a feature of one embodiment which has the same or asimilar function as another feature of another embodiment can beexchanged. In particular, a feature of one embodiment which adds anadditional function to another embodiment can be added to said otherembodiment.

The present invention relates to a method for tracking trackable objectsusing a tracking device during a medical workflow comprising a pluralityof workflow steps, wherein each trackable object has at least onemarker. One step of the method involves acquiring a set of trackingdevice positions, wherein each tracking device position is associatedwith at least one workflow step. Another step involves identifying aworkflow. Yet another step involves sequentially and automaticallymoving the tracking device to the tracking device positions associatedwith the workflow steps. According to the present invention, the actualtracking step is performed only when the tracking device is in a fixedposition, i.e. when the camera is not moving.

The tracking device can be a tracking camera or an EM transmitter. EMtracking systems using an EM transmitter and one or more EM receiversacting as markers on objects are widely known, such that a detaileddescription is omitted. The EM transmitter can be at least one antennaor a combination of at least one antenna with transmitter electronics.In the following, the tracking device is always described as being acamera and the tracking device position is referred to as cameraposition. However, the invention is further equally applicable to atracking device being an EM transmitter, such that the camera positioncorresponds to an EM transmitter position. The tracking volume of atracking camera typically is the field of view of the camera, while thetracking volume of an EM transmitter typically is a spatial region whichis permeated by the EM field generated by the EM transmitter.

Within the present invention, the tracking camera can assume severalcamera positions, and a camera position to be assumed is defined for aparticular workflow step. The camera position assigned to a particularstep of the medical workflow is preferably an optimum position regardingthe accuracy with which the objects used during this workflow step aretracked. The set of camera positions preferably comprises two or more,three or more or five or more camera positions. In other words, thecamera assumes two or more, three or more or five or more camerapositions during the workflow.

Automatically moving the camera means that no physical manipulation ofthe camera by a human being is required in order to move the camera intoa particular position. The camera can for example be a motorised camera.In this document, the term “position” means the spatial location in upto three translational dimensions and/or the alignment in up to threerotational dimensions.

Performing the tracking step only when the camera is in a fixed positionmeans that any inaccuracies in the tracking result introduced by themovement of the camera during the tracking process are eliminated, whilethe camera can still be positioned for each workflow step in order toachieve a sufficient tracking result.

It is the function of a marker to be detected by a marker detectiondevice (for example, a camera or an ultrasound receiver), such that itsspatial position (i.e. its spatial location and/or alignment) can beascertained. The detection device is in particular part of a navigationsystem. The markers can be active markers. An active marker can forexample emit electromagnetic radiation and/or waves, wherein saidradiation can be in the infrared, visible and/or ultraviolet spectralrange. The marker can also however be passive, i.e. can for examplereflect electromagnetic radiation in the infrared, visible and/orultraviolet spectral range. To this end, the marker can be provided witha surface which has corresponding reflective properties. It is alsopossible for a marker to reflect and/or emit electromagnetic radiationand/or waves in the radio frequency range or at ultrasound wavelengths.A marker preferably has a spherical and/or spheroid shape and cantherefore be referred to as a marker sphere; markers can also, however,exhibit a cornered for example, cubic shape.

A marker device can for example be a reference star or one or more(individual) markers in a predetermined spatial relationship. A markerdevice comprises one, two, three or more markers in a predeterminedspatial relationship. This predetermined spatial relationship is inparticular known to a navigation system and for example stored in acomputer of the navigation system.

The expression “acquiring” data, such as the set of camera positions,encompasses in particular (within the framework of a data processingmethod) the scenario in which the data are determined by the dataprocessing method or program. The meaning of acquiring data inparticular also encompasses the scenario in which the data are receivedor retrieved by the data processing method or program, for example fromanother program, a previous method step or a data storage medium, inparticular for further processing by the data processing method orprogram. Thus, “acquiring data” can also for example mean waiting toreceive data and/or receiving the data. The received data can forexample be inputted via an interface. Acquiring data can also mean thatthe data processing method or program performs steps in order to(actively) receive or retrieve the data from a data source, for instancea data storage medium (such as for example a ROM, RAM, database, harddisc, etc.) or via the interface (for instance, from another computer ora network). The data can be made “ready for use” by performing anadditional step before the acquiring step. In accordance with thisadditional step, the data are generated in order to be acquired. Thedata are in particular detected or captured (for example, by ananalytical device). Alternatively or additionally, the data are inputtedin accordance with the additional step, for instance via interfaces. Thedata generated can in particular be inputted (for instance, into thecomputer). In accordance with the additional step (which precedes theacquiring step), the data can also be provided by performing theadditional step of storing the data in a data storage medium (such asfor example a ROM, RAM, CD and/or hard drive), such that they are readyfor use within the framework of the method or program in accordance withthe invention. Thus, acquiring data can also involve commanding a deviceto obtain and/or provide the data to be acquired.

The camera position is preferably verified, for example after the cameraposition has been changed or before the first workflow step isperformed. This verification can be based on at least one marker whichis in a fixed position, for example attached to an item of operatingroom infrastructure such as an operating room table. If the cameraposition is to be verified after it has been changed, it can be verifiedrelative to at least one marker or a marker device which has not changedits position during the movement of the camera. The change in theposition of the camera can be calculated from the relative positions ofthe camera and the at least one marker or the marker device for twodifferent camera positions.

There are numerous ways of determining a camera position within the setof camera positions. One option is to manually enter a position into theset of camera positions. Among other implementations, the up to sixdimensions of the position can be entered manually, for example asco-ordinates and/or angles, for example as numerical values. Anotheroption would be to position the camera by controlling the cameraposition using an input device, such as a keyboard, a joystick, a mouse,a touch screen or any other suitable device, and then using the currentposition of the camera as a position in the set of camera positions.

Another option is to determine a position to be included the set ofcamera positions automatically. This preferably includes the steps of:sequentially moving the camera to a plurality of test positions, eitherautomatically or manually; storing a test position as a candidateposition if all the markers of all the trackable objects used within aparticular workflow step are within the tracking volume of the camera;and selecting the candidate position in which the distances from themarkers to the centre of the tracking volume are minimised as a positionto be included in the set of camera positions. This utilises the factthat the tracking accuracy of markers in a central portion of thetracking volume is typically higher than the tracking accuracy ofmarkers in a peripheral portion of the tracking volume.

The criterion of the minimised distance can be implemented in a numberof ways. Example options include minimising the average distance orminimising the weighted average distance.

As an alternative to or in addition to the criterion of minimising thedistance from the markers to the centre of the tracking volume, it isalso possible to use the criterion of selecting the highest possiblecandidate position. If, for example, several candidate positions resultin the same minimum distance, then the candidate position with thehighest possible position is selected. A high position of the trackingcamera reduces the probability of an obstacle obstructing the line ofsight between the camera and a marker.

Another option is to select the candidate position in which the anglesbetween the line of sight of the camera and normal vectors which arenormal to a plane in which the markers of a trackable object lie areminimised. This utilises the fact that the tracking accuracy for anobject is at its highest if the markers do not overlap in the outputtedimages of the tracking camera.

In another embodiment, a test position is stored as a candidate positionif all the markers of all the trackable objects used within a particularworkflow step are within a predetermined sub-volume of the trackingvolume. This sub-volume is preferably a central portion of the trackingvolume, in which the tracking accuracy is at its highest. Within thisembodiment, the number of candidate positions is reduced, such thatfewer candidate positions have to be analysed in order to find theposition which is to be stored as a position within the set of camerapositions.

In yet another embodiment, priorities are assigned to the trackableobjects used within a particular workflow step, and a test position isstored as a candidate position if all the markers of objects with apriority which is higher than a threshold value are within apredetermined sub-volume of the tracking volume. This embodiment ensuresthat at least the objects with a priority which is higher than thethreshold value can be tracked to a desired level of accuracy. This isparticularly useful if there is no test position of the tracking camerain which all the markers of all the objects to be tracked lie within thesub-volume of the tracking volume.

Additionally or alternatively, priorities assigned to the trackableobjects are used to select one of the candidate positions for inclusionin the set of camera positions.

According to another embodiment, a set of candidate positions is storedfor each of a plurality of workflow steps, and the number of (preferablyconsecutive) workflow steps which comprise the same candidate positionin their respective sets of candidate positions is determined. Thecandidate position which features the highest number of times in theworkflow steps is selected as a position to be included in the set ofcamera positions. This approach reduces the number of times the camerahas to be repositioned, at the possible expense that the camera positionmight not be the best possible camera position for a particular workflowstep.

In another embodiment, the method comprises the additional step ofproviding indication information which indicates that no suitable cameraposition has been found and that the camera should be repositioned. Theindication information is preferably represented by an indicationinformation signal, which can be optical, acoustic and/or tactile innature. Repositioning the camera means that a base, such as a tripod,relative to which the camera position can be selected, is changed, forexample by moving the base to another position. This new position of thecamera base enables camera positions other than those of the previouscamera base position, such that a test position may then be found inwhich all the criteria for using this test position in the set of camerapositions are fulfilled. In another case, the location of the camera isfixed and only the orientation of the camera is adjustable. In thiscase, the camera has to be relocated.

In one embodiment, the medical workflow is identified manually byinputting identification data which identify a workflow, for example byselecting the medical workflow from a list of medical workflows.However, the medical workflow is preferably identified automatically.

The medical workflow is for example identified by identifying trackableobjects. This can mean identifying the type of an object or identifyinga unique object. Since every medical workflow requires a particular setof objects, the medical workflow can be determined from the objectspresent.

Upon identifying trackable objects, the camera preferably assumes aplurality of positions to enhance its detection volume, wherein thedetection volume is a spatial region in which markers can be detectedfor the current camera position. In particular, the camera positions arechosen such that the detection volumes for these camera positions(slightly) overlap. The enhanced detection volume preferably comprisesthe whole operation room environment. In one embodiment, the camera hasa restricted range of motion limiting possible camera positions. Atleast some of the camera positions are preferably on the border of thisrange of motion.

An object is preferably identified using the tracking camera, forexample by identifying the shape of an object using image processing orby determining the configuration of markers connected to an object.Additionally or alternatively, objects are identified using RFID chipsattached to them.

A trackable object can be an anatomical structure, such as a bone or apart of a bone, or a medical instrument, such as a scalpel, a cuttingblock, a clamp, a treatment beam generator, a C-arm, an ultrasoundtransducer or an imaging device such as an x-ray, CT or MRT device.

In one embodiment, a camera position is corrected on the basis of theposition of a marker attached to a body or to a part of a body to betreated. The marker, which can also be a plurality of markers, thus actsas a reference. Within this embodiment, the camera position can beadjusted if the body is in a position which differs from an (expected)position on which the camera position within the set of camera positionsis based. The motivation for this embodiment is that the trackableobjects are typically in positions relative to the body, and notnecessarily relative to a reference system in which the camera positionswithin the set of camera positions are typically defined. Alternatively,the camera positions within the set of camera positions are definedrelative to the reference.

In another embodiment, the tracking device position is corrected on thebasis of the detectability of the markers. In the case of a camera, thecamera position is corrected on the basis of the visibility of themarkers. This means that the camera is first moved into the positionspecified in the set of camera positions and the visibility of themarkers is then determined. If the required markers are not all visible,then the camera position is (preferably automatically or upon manualactivation) adjusted until all the required markers are visible. Amanual activation ensures that the camera does not perform sudden andunexpected movements. In general, the description given for the camerais applicable in analogy to any other tracking device, such as an EMtransmitter.

The method in accordance with the invention is in particular a dataprocessing method. The data processing method is preferably performedusing technical means, in particular a computer. In particular, the dataprocessing method is executed by or on the computer. The computer inparticular comprises a processor and a memory in order to process thedata, in particular electronically and/or optically. The calculatingsteps described are in particular performed by a computer. Determiningsteps or calculating steps are in particular steps of determining datawithin the framework of the technical data processing method, inparticular within the framework of a program. A computer is inparticular any kind of data processing device, in particular electronicdata processing device. A computer can be a device which is generallythought of as such, for example desktop PCs, notebooks, netbooks, etc.,but can also be any programmable apparatus, such as for example a mobilephone or an embedded processor. A computer can in particular comprise asystem (network) of “sub-computers”, wherein each sub-computerrepresents a computer in its own right. The term “computer” alsoencompasses a cloud computer, in particular a cloud server. The term“cloud computer” also encompasses a cloud computer system which inparticular comprises a system of at least one cloud computer, inparticular a plurality of operatively interconnected cloud computerssuch as a server farm. The cloud computer is preferably connected to awide area network such as the world wide web (WWW). Such a cloudcomputer is located in a so-called cloud of computers which are allconnected to the world wide web. Such an infrastructure is used for“cloud computing” which describes computation, software, data access andstorage services which do not require the end user to know the physicallocation and configuration of the computer which delivers a specificservice. In particular, the term “cloud” is used as a metaphor for theinternet (world wide web). The cloud in particular provides computinginfrastructure as a service (IaaS). The cloud computer may function as avirtual host for an operating system and/or data processing applicationwhich is used for executing the method of the invention. The cloudcomputer is preferably an Elastic Compute Cloud (EC2) as provided byAmazon Web Services™. A computer in particular comprises interfaces inorder to receive or output data and/or perform an analogue-to-digitalconversion. The data are in particular data which represent physicalproperties and/or are generated from technical signals. The technicalsignals are in particular generated by means of (technical) detectiondevices (such as for example devices for detecting marker devices)and/or (technical) analytical devices (such as for example devices forperforming imaging methods), wherein the technical signals are inparticular electrical or optical signals. The technical signalsrepresent in particular the data received or outputted by the computer.

The invention also relates to a program which, when running on acomputer or when loaded onto a computer, causes the computer to performone or more or all of the method steps described herein and/or to aprogram storage medium on which the program is stored (in particular ina non-transitory form) and/or to a computer on which the program isrunning or into the memory of which the program is loaded and/or to asignal wave, in particular a digital signal wave, carrying informationwhich represents the program, in particular the aforementioned program,which in particular comprises code means which are adapted to performany or all of the method steps described herein.

Within the framework of the invention, computer program elements can beembodied by hardware and/or software (this includes firmware, residentsoftware, micro-code, etc.). Within the framework of the invention,computer program elements can take the form of a computer programproduct which can be embodied by a computer-usable, in particularcomputer-readable data storage medium comprising computer-usable, inparticular computer-readable program instructions, “code” or a “computerprogram” embodied in said data storage medium for use on or inconnection with the instruction-executing system. Such a system can be acomputer; a computer can be a data processing device comprising meansfor executing the computer program elements and/or the program inaccordance with the invention, in particular a data processing devicecomprising a digital processor (central processing unit (CPU)) whichexecutes the computer program elements and optionally a volatile memory(in particular, a random access memory (RAM)) for storing data used forand/or produced by executing the computer program elements. Within theframework of the present invention, a computer-usable, in particularcomputer-readable data storage medium can be any data storage mediumwhich can include, store, communicate, propagate or transport theprogram for use on or in connection with the instruction-executingsystem, apparatus or device. The computer-usable, in particularcomputer-readable data storage medium can for example be, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infraredor semiconductor system, apparatus or device or a medium of propagationsuch as for example the internet. The computer-usable orcomputer-readable data storage medium could even for example be paper oranother suitable medium onto which the program is printed, since theprogram could be electronically captured, for example by opticallyscanning the paper or other suitable medium, and then compiled,interpreted or otherwise processed in a suitable manner. The datastorage medium is preferably a non-volatile data storage medium. Thecomputer program product and any software and/or hardware described hereform the various means for performing the functions of the invention inthe example embodiments. The computer and/or data processing device canin particular include a guidance information device which includes meansfor outputting guidance information. The guidance information can beoutputted, for example to a user, visually by a visual indicating means(for example, a monitor and/or a lamp) and/or acoustically by anacoustic indicating means (for example, a loudspeaker and/or a digitalspeech output device) and/or tactilely by a tactile indicating means(for example, a vibrating element or vibration element incorporated intoan instrument).

The present invention also relates to a tracking system for trackingtrackable objects using a camera during a medical workflow comprising aplurality of workflow steps, wherein each trackable object has at leastone marker. The tracking system comprises a tracking camera and acontrol unit. The control unit is adapted to acquire a set of camerapositions, each of which is associated with at least one workflow step,in order to identify a workflow, sequentially and automatically move thecamera to the camera positions associated with the workflow steps, andperform tracking only when the camera is in a fixed position.

In general, the control unit is adapted to carry out or initiate themethod steps as explained above.

The present invention shall now be explained in more detail withreference to the accompanying figures. These figures show:

FIG. 1 a schematic structure of a medical tracking system;

FIG. 2 an example operating room configuration.

FIG. 1 schematically shows the structure of a medical tracking system 1.The tracking system 1 comprises a CPU (central processing unit) 2connected to an image pick-up unit 3, an input device 4, a display unit5 and a memory unit 6. The image pick-up unit 3 comprises a base (notshown), a drive unit 3 a and a camera 3 b. The base of the image pick-upunit 3 is placed at a fixed position. The drive unit 3 a is configuredto drive the camera 3 b relative to the base of the image pick-up unit3, such that the camera 3 b can assume a plurality of positions. Thedrive unit 3 a could include a robotic arm.

The CPU 2 is configured to support a medical workflow comprising aplurality of workflow steps by enabling trackable objects to be tracked.To this end, the memory unit 6 stores a set of camera positions, whereineach camera position is associated with at least one workflow step. Morethan one workflow step can be associated with a particular cameraposition.

Depending on the workflow step, the CPU 2 determines a camera positionfrom the set of camera positions in the memory unit 6 which correspondsto the workflow step and instructs the drive unit 3 a of the imagepick-up unit 3 to move the camera 3 b to the determined camera position.Alternatively, the CPU 2 provides a control signal to the drive unit 3 awhich activates the drive unit 3 a as required.

The CPU 2 is also configured to interpret the output signal of thecamera 3 b in order to detect the positions of markers within the fieldof view, also referred to as the tracking volume, of the camera 3 b.When a plurality of markers, preferably three or four markers, areattached to an object in a known configuration, the position of theobject can be calculated from the positions of its markers. The CPU 2can thus track objects. The positions of the tracked objects could bedisplayed on the display unit 5 and/or used in a medical navigationprocess.

In order to determine the camera position to be set, the CPU 2 needs toknow the workflow and the workflow step. Information regarding theworkflow and the workflow step can be inputted using the input device 4,which can be a touch-sensitive surface of the display unit 5 or anyother suitable input means. Alternatively, the CPU 2 can determine theobjects present in the operating room by analysing the output signal ofthe camera 3 b (or any other camera) and then determine the workflowfrom the set of objects present. For this purpose, the memory unit 6 canstore a set of workflows and lists of objects to be used in eachworkflow. From the current positions of the objects, the CPU 2 can alsodetermine the current workflow step and accordingly the associatedcamera position. The workflow step, in particular the transition fromone workflow step to the next, can also be determined by the CPU 2 byanalysing the output signal of the camera 3 b (or any other camera) anddetecting a gesture which is performed by a person such as a surgeon andindicates to the tracking system 1 that the next workflow step is to beperformed or indicates which workflow step is to be performed next.

FIG. 2 schematically shows a scenario corresponding to one step of amedical workflow. In this scenario, a patient 7 is lying on an operatingroom table 11. In the corresponding workflow step, an x-ray image of thepatient 7, in particular of the patient's head, is to be acquired usingan x-ray imaging unit 9. In order to analyse the x-ray image, it isessential to know the viewing direction from which the image wasrecorded. To this end, a marker device 8 comprising three markers in aknown configuration is rigidly attached to the patient 7, in particularto the patient's head. A marker device 10 comprising three markers in aknown configuration is attached rigidly to the x-ray imaging unit 9. Ifthe relative position between the marker devices 8 and 10, and thereforebetween the patient 7 and the x-ray imaging unit 9, is known, then theviewing direction of an x-ray image of the patient 7 generated by thex-ray imaging unit 9 is also known. It is thus advantageous to track thetrackable objects, i.e. the patient 7 and the x-ray imaging unit 9 usingthe medical tracking system 1.

In order to track the patient 7 and the x-ray imaging unit 9, the markerdevices 8 and 10 need to be within the tracking volume of the trackingcamera 3 b. The camera 3 b therefore needs to be placed in a position inwhich this requirement is fulfilled. As explained above, the camera 3 bis automatically moved to such a position, which belongs to a set ofcamera positions and is associated with a workflow step according to thescenario shown in FIG. 2. How this camera position is determined andstored in the set of camera positions will now be described withreference to this figure.

In FIG. 2, the tracking volume of the camera 3 b is indicated as aconical region. In order to determine the camera position, the camera 3b is moved into three test positions, wherein the tracking volumes forthe first and third test position are indicated by a dashed boundaryline and the tracking volume for the second test position is indicatedby a continuous boundary line. The first test position corresponds tothe upper tracking volume, while the third test position corresponds tothe lower tracking volume.

A set of candidate positions is determined from the set of testpositions. A test position is added to the set of candidate positions ifall the markers of all the objects to be tracked in the correspondingworkflow step are within the tracking volume of the camera 3 b when thecamera 3 b is in said test position. In the present example, if thecamera 3 b is in the first test position or in the third test position,the markers of the marker devices 8 and 10 do not all lie within thetracking volume. The first and third test positions are therefore notadded to the set of candidate positions. In the second test position,the markers of the marker devices 8 and 10 all lie within the trackingvolume of the camera 3 b, such that the second test position is added tothe set of candidate positions.

In the present case, after all the test positions have been tested, theset of candidate positions only comprises one camera position, i.e. thesecond test position. This camera position is thus added to the set ofcamera positions and associated with the workflow step. If the set ofcandidate positions comprises more than one camera position, one or moresuitable criteria are applied in order to select one candidate positionas the camera position to be added to the set of camera positions.

When the medical workflow is performed, one workflow step corresponds tothe scenario shown in FIG. 2. When this workflow step is reached, thecamera position associated with this workflow step is determined fromthe set of camera positions, and the camera 3 b is automatically movedinto this camera position. The patient 7 and the x-ray imaging unit 9are only tracked once the camera 3 b has reached the determined cameraposition and are not tracked while the camera is moving.

It should be noted that the tracking method described herein does notrelate to or comprise any surgical step. In particular, attaching amarker or marker device to a (human) body is not a part of the presentinvention. The present invention is based on a state in which a markeror marker device has already, i.e. previously, been attached. Thisinvention relates solely to tracking trackable objects, positioning thecamera of a tracking system and determining a camera position to beassociated with at least one workflow step.

1. A method for tracking trackable objects using a medical trackingdevice, the tracking device being a tracking camera or an EMtransmitter, during a medical workflow comprising a plurality ofworkflow steps, wherein each trackable object has at least one markerand the method comprises the steps of: acquiring a set of trackingdevice positions, wherein each tracking device position is associatedwith at least one workflow step; identifying a workflow; sequentiallyand automatically moving the tracking device to the tracking devicepositions associated with the workflow steps; and performing a trackingstep only when the tracking device is in a fixed position.
 2. The methodaccording to claim 1, wherein the tracking device covers a trackingvolume in each position, and a position for inclusion in the set oftracking device positions is determined automatically by: sequentiallymoving the tracking device to a plurality of test positions; storing atest position as a candidate position if all the markers of all thetrackable objects used within a particular workflow step are within thetracking volume of the tracking device; and selecting the candidateposition in which the distances from the markers to the centre of thetracking volume are minimised as a position to be included in the set oftracking device positions.
 3. The method according to claim 2, whereinthe highest possible candidate position is selected.
 4. The methodaccording to claim 2, wherein, if the tracking device is a camera, thecandidate position in which the angles between the line of sight of thecamera and normal vectors which are normal to a plane in which themarkers of a trackable object lie are minimised.
 5. The method accordingto claim 2, wherein a test position is stored as a candidate position ifall the markers of all the trackable objects used within a particularworkflow step are within a predetermined sub-volume of the trackingvolume.
 6. The method according to claim 2, wherein priorities areassigned to the trackable objects used within a particular workflowstep, and a test position is stored as a candidate position if all themarkers of objects with a priority which is higher than a thresholdvalue are within a predetermined sub-volume of the tracking volume. 7.The method according to claim 2, further comprising the step ofproviding indication information which indicates that no suitabletracking device position has been found and that the tracking deviceshould be repositioned.
 8. The method according to claim 1, wherein themedical workflow is identified automatically.
 9. The method according toclaim 8, wherein the medical workflow is identified by identifyingtrackable objects used in the workflow.
 10. The method according toclaim 1, wherein a tracking device position is corrected on the basis ofthe position of a marker attached to a body or to a part of a body to betreated.
 11. The method according to claim 1, wherein a position isentered into the set of tracking device positions manually.
 12. Themethod according to claim 1, wherein the tracking device position iscorrected on the basis of the detectability of the markers.
 13. Acomputer program embodied on a non-transitory computer readable mediumwhich, when running on a computer or when loaded onto a computer, causesthe computer to acquire a set of tracking device positions, wherein eachtracking device position is associated with at least one workflow step;identify a workflow; sequentially and automatically move the trackingdevice to the tracking device positions associated with the workflowsteps; and performing a tracking step only when the tracking device isin a fixed position.
 14. A tracking system for tracking trackableobjects having markers during a medical workflow comprising a pluralityof workflow steps, said system comprising a tracking device being atracking camera or an EM transmitter and a control unit which is adaptedto: acquire a set of tracking device positions, wherein each trackingdevice position is associated with at least one workflow step; identifya workflow; sequentially and automatically move the tracking device tothe tracking device positions associated with the workflow steps; andperform a tracking step only when the tracking device is in a fixedposition.
 15. (canceled)